ATP, in addition to its function as an intracellular energy donor, is now recognized as an important neurotransmitter or cotransmittor, in both the central and peripheral nervous system (Ralevic, V., et al., Pharmacol. Rev., 50:413-492 (1998)). ATP is released from a variety of cell types, including nerve fibers, upon stimulation and produces diverse effects on many tissues by activation of specific membrane receptors known as P2 receptors (Burnstock, G., Pharmacol. Rev., 24:509-581 (1972); Burnstock, G., Cell Membrane Receptors for Drugs and Hormones: A Multidisciplinary Approach, edited by R. W. Straub and L. Bolid. New York: Raven, 1978, p. 107-118).
P2 receptors are further subdivided into P2X and P2y receptors. P2X and P2y receptors have similar agonist potency, but differ in their molecular structure and associated transduction mechanisms. P2X receptors are ligand-gated ion channels with two membrane spanning domains, whereas P2Y receptors are G-protein coupled receptors with typically seven membrane-spanning domains (Abbracchio, M. P., et al., Pharmacol. Ther., 64:445-475 (1994); Fredholm, B. B., et al., Pharmacol. Rev., 46:143-156 (1994)).
P2X7 purinergic receptor is a ligand-gated cation channel activated by extracellular ATP. Under physiological conditions it is selectively permeable to small cations. Sustained stimulation by ATP causes formation of a large transmembrane pore permeable to hydrophilic molecules up to 900 Da resulting in the induction of apoptosis.
The P2X7 receptors were originally cloned from brain tissue and were suggested to be localized only to activated microglia in the regulation of inflammatory responses (interleukin-1β and caspase-1 activation). Activated microglia have been observed in patients suffering from many CNS disorders including cerebral ischaemia, Parkinson's, Alzheimer's and Huntigton's diseases. Recently P2X7 was localized to hippocampal neurons and shown to regulate the release of GABA and glutamate further implicating its role in neurodegeneration and neurodegenerative diseases. P2X7 receptors are involved in microglial and astrocyte activation, excitotoxicity, nitric oxide and cytokine secretion and recent neuronal localization make them an important therapeutic target.
The central role of purinoceptors, particularly P2X7 receptors, in regulating such diverse brain processes make such receptors particularly important targets for therapeutic development. Thus, there is a clear need for the identification and characterization of such receptors, in addition to, their association to disease states and/or processes. In particular, there is a need to isolate and characterize additional purinoreceptors.